1. Field of the Invention
This invention provides an apparatus for cooling nuclear magnetic resonance probe coils. The apparatus finds use in NMR spectrometers and microscopes that utilize superconducting probe coils for detection of magnetic signals.
2. Description of Related Art
In NMR spectroscopy and microscopy, a sample to be analyzed is positioned within the bore of a high field magnet. A probe for detecting magnetic fields is positioned around the sample. The probe includes a means for positioning a detector coil near the sample and a detection apparatus for detecting the magnetic moments of sample material that has been subjected to the external field of the high field magnet. The detection apparatus may be a series of radio frequency coils, such as are described in U.S. Pat. No. 3,771,055, or pairs of Helmholtz coils positioned on opposite sides of the sample. Typically, the receiver coils are made of copper or other ordinary conductor.
The sensitivity of NMR spectrometers is limited by noise from the receiver coil. The noise from copper receiver coils can be reduced by cooling of the coils. Cryogenic cooling, that is, cooling far below room temperature, particularly below 100 K and as low as the temperature of liquid helium, approximately 4 K, can thus improve the performance of conventional copper NMR receiver coils. The use of superconducting receiver coils can potentially provide significant improvement in signal to noise ratios over conventional coils operated at low temperatures. Superconducting detectors are desirable because their low resistance and extremely low noise levels make it possible to design systems capable of detecting extremely small magnetic signals. Although superconducting magnets have been used in commercial NMR systems for over twenty years, interest in their use in receiver coils is recent.
Superconductors become superconducting when cooled below their critical temperature, but require temperatures well below the critical temperature, typically to about one-half (1/2) their critical temperature or lower, to achieve the necessary performance. Early superconductors only became superconducting at extremely low temperatures, necessitating the use of liquid helium for operation. With the discovery in 1986 of high temperature superconductors (HTS), it was recognized that the signal-to-noise ratio of magnetic resonance instruments might be further improved by the use of HTSs as the magnetic field detectors in NMR probes. Realization of this potential, however, requires a practical means of maintaining the receiver coil at a constant low temperature because the resonant frequency is temperature dependent. For example, YBCO with a critical temperature of about 90 K must be cooled to 10-60 K for acceptable performance in an NMR coil.
Superconducting NMR receiver coil design is further limited by the requirements for thin-film HTS coils. For both nuclear magnetic spectroscopy and nuclear magnetic microscopy, the superconducting coil is deposited as a thin film of an oxide superconductor (such as YBCO) on a crystalline substrate (typically sapphire or LaAlO.sub.3). U.S. Pat. No. 5,258,710, which has a common inventor and which is incorporated herein by reference, describes several designs for HTS detectors for NMR microscopy. More recently, "HTS receiver coils for magnetic-resonance instruments," Withers et al, SPIE vol. 2156, p 27, Jan. 94 described HTS microscopy probe coils. Because the coil is a thin film on a flat substrate, it cannot be formed around a sample space, nor can it be shaped into a hollow tube or similar structure.
Previous attempts to maintain probe coils at cryogenic temperatures have required the use of liquid helium or liquid nitrogen. P. Styles et al., "A High-Resolution NMR Probe in Which the Coil and Preamplifier Are cooled with Liquid Helium," Journal of Magnetic Resonance 60:397-404 (1984) described an apparatus for cooling a copper coil, its preamplifier and tuning gear with liquid helium. A liquid nitrogen jacketed helium dewar was fabricated to fit within the magnet and hold the amplification components. The Helmholtz coil receiver consisted of a pair of copper tubes positioned around the bore for receiving the sample tube. Liquid helium was pumped through the center of the tubes. Styles et al. reported an increase in the coil's Q factor from 150 to 1000. This apparatus is, however, unsuitable for use with HTS thin film coils because it requires tubular coils not realizable with thin-film superconductors on a crystalline substrate.
U.S. Pat. No. 5,258,710, issued to Robert Black, a coinventor herein, used a multi-walled quartz dewar to hold a superconducting probe coil. Nitrogen gas was cooled to liquid nitrogen temperature by passing it through a liquid nitrogen bath, and then flowed into a space in the dewar where it cooled the outer walls. Helium gas was delivered to a heating block within the dewar, and then allowed to flow through a capillary onto the coil. It, like the previous design, has the disadvantage of requiting supplies of liquid nitrogen and liquid helium to maintain the cold probe. The design has the additional disadvantage of requiting fabrication of a complex quartz dewar. The dewar design is difficult and costly to implement.
U.S. Pat. No. 5,247,256, issued to Marek, describes an rf receiver coil arrangement for NMR spectrometers in which the rf coil is in contact and cooled by a cooled platform. The patent describes cooling the platform, which may be sapphire or quartz in thermal contact with copper using a liquid helium or nitrogen stream. While the patent discloses one embodiment utilizing a closed circuit cooling system with integrated helium recovery within the body of the probe, it lacks any suggestion of a need for an efficient heat exchange mechanism, or minimization of parasitic thermal load, requires a liquid helium or nitrogen cryogen rather than a gaseous one, and, in its closed circuit embodiment incorporates the entire closed circuit system within the body of the probe.